1. Field of the Invention
The present invention relates to a projection optical system, and more specifically, to an oblique projection optical system that has a reflective optical element with an optical construction suitable for rear projection and that performs enlargement projection in an oblique direction from a primary image surface to a secondary image surface.
2. Description of Related Art
Examples of an oblique optical system employing a reflective optical element for wide-angle enlargement projection from the primary image surface on the reduction side to the secondary image surface on the enlargement side include those proposed in patent documents 1 to 3. The oblique projection optical systems as described in patent documents 1 to 3 employs a reflective optical element having a positive optical power and a reflective optical element having a negative optical element to achieve slimming-down of a rear projection apparatus (i.e., slimming-down of the apparatus thickness in the direction normal to the screen surface).                [Patent Document 1] U.S. Pat. No. 6,779,897        [Patent Document 2] U.S. Pat. No. 5,871,266        [Patent Document 3] Japanese Patent Application Laid-open No. 2003-255229        
However, just use of a reflective optical element having a positive optical power and a reflective optical element having a negative optical element, as described in patent documents 1 to 3, suffers from limitations in achieving the slimming-down of a rear projection apparatus in terms of correction of aberration such as curvature of field and distortion. Moreover, the use of a large reflective optical element in a typical oblique projection optical system that employs a reflective optical element can reduce the curvature of field and the distortion; however, the use of a large reflective optical element causes cost increase or upsizing of the entire optical system.